Amplifier



CZv S. ROOT Sept. 17, 1940.

AMPLIFIER Inverwtor;

10 y Hi Filed May 51, 1959 .m y o e R W m S AW e f m H a h C respect toground on its two conductors.

Patented Sept. 1 7 i940 PATENT OFFICE AMPLIFIER Charles S. Root,Bridgeport, Conn, assignor .to General Electric Company, a. corporationof New York Application May 31, 1939, Serial No. 276,597

9 Claims. (01. 179 171) invention relates to amplifiers. It has for oneof its objects to provide certain improve-. ments in amplifiers employedbetween unbalanced and balanced circuits whereby the efiects" ofdegeneration and-undesired noise due to energizing-currents are reduced.

One common form of coupling between unbalanced and balanced circuitsinvolves an ampli-- fier in which the external anode cathodeimpedance-is divided in two equal impedances by a ground connection. Thebalanced circuit is then connectedbetween the anode and cathode and thusreceives equal and opposite voltages with The unbalanced'circuit isconnected between grid and ground.

My invention has to do with such circuits and it has for one of itsobjects to reduce the degeneration produced by the impedance which isconnected between the cathode and ground in such an amplifier.

A further object of my invention is to equalize the hum currentssupplied to the balanced circult by reason of the alternating cathodeenergizing currents of the amplifier so that such currents may bebalanced out in that circuit.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My invention,itself, "however, both as to its organization and method of operation,together with further objects' and advantages thereof, may best beunderstood byreference to 'the following description takenin connectionwith the accompanying 9- drawing' in which the single figure representsan embodiment of my invention.

Referring to the drawing I have shown therein an amplifier I connectedto amplify signals received upon a circuit 2 and to supply such signalsto the input circuit of an amplifier 3. The amplifier 3 is one having anoutput circuit comprising resistances 4 and 5 connected in series, theresistance 4 being connected between the anode'and' positive terminal ofthe anode energizing source indicated at +3 and the resistance 5beingconnected between the cathode and the negative terminal B of thissource, the two resistancesbeing proportioned so that the volt- I agesthereon are equal. 7 This negative terminal -B is grounded as indicatedat 6 whereby the anode, andnqathode of the device 3 operate with 1 equalsignal ,voltages thereon with respect to .1 ground, these voltages beingin opposite phase relation. Oscillations appearing on the resistance 4are supplied to'the input circuit of an amplifier I fthro'uglratransformer 9 to an output circuit I0. 1

Resistances I I and I2 comprise the usual grid resistances for theseamplifiers I and 8 and simi larly condensers I3 and I4 comprise theusual coupling capacitors for these amplifiers whereby the grid ofamplifier l is coupled to the anode of amplifier 3 and the grid ofamplifier 8 is coupled to the cathode of amplifier 3.

The resistor I5 shunted by condenser I6 is the usual cathode biasresistance common to the grid and anode circuits of amplifiers 'I and 8for pro-.

ducing bias upon the grids of these amplifiers.

Also resistance Il, shunted by condenser I8, is .7

connected in common with the grid and anode circuits of. amplifier 3 forproducing bias voltage f cathode terminal of-resistance 5. Theresistance 23 is also connected between the grid and-the cathodeterminal of resistance 5.

Condensers I9 and 22 have large capacity and very low impedance attheirequency' of currentsto be considered herein, namely, audiofrequency currents which it is desired that the system shall amplify andalso hum currents, which may be of sixty and one hundred and twentycycles frequency, which it is desired to eliminate. Resistors 2| and 5are thus effectively connected in parallel, the upper terminals oftheseresistors being connected together by condenser 22 and the lowerterminals being connected together'by condenser 24, which is of similarhigh capacity and: This latter condenser may be low impedance. one ofthe usual plate smoothing or filter capacitors of the power supplysystem. Similarly resistances and 23 are effectively connected inparallel for currents having the frequency herein to be considered.

by a connection through condenser 22 to the In the operation of thesystem, currents which are to be amplified appear in the output circuitof amplifier 3 and electromotive. force produced thereby on resistors 4and 5 are supplied in opposite phase through condensers I3 and I4 to fthe grids of the two amplifiers I and 8.

In order to secure maximum amplification of the discharge device 3 it isdesirable 'to'reduce insofar as possible the efiect, of degenerationcaused by resistance 5, that is, if the signal voltage on resistance 5be impressed upon the grid of amplifier 3 in opposed phase to thevoltage; appearing there from amplifier I, it tends to re-,

connected as. shown. In this, way the voltage which appears uponresistances 5 and 2| now connected in parallel, can be supplied betweenthe grid and cathode only through the internal impedance of dischargedevice I. This internal impedance may be made very high by the choice ofdischarge device employed at l and by the proper selection of voltagesapplied thereto. For example, this discharge device may be one of thescreen grid, or screen and suppressor grid, type and if desired theanoderesistance may be materially increased by reducing the voltagesupplied to the screen grid thereof to a suitably low value. Thisvoltage, as shown in the drawing, is supplied to the screen grid 25throughresistance 26 from the positive voltage source +B.

It has been found that with the connections shown, by making theresistance of discharge device high and the parallel resistance ofresistors 20 and 23 as low as practicable, practically the fullamplification of discharge device 3 may be taken advantage of, theeiTect of degeneration being reduced to a very low value.

This arrangement is also effective in reducing undesired, or humcurrentsproduced by reason of the-cathode energization of the amplifier 3. Itwill be observed that the amplifiers shown, as in common practice, areof the unipotential type energized with alternating current from asuitable cathode energizing circuit 2'! which is commonly supplied withalternating voltage of about 6 volts. Leakage resistance commonly existsbetween heater and cathode in these tubes so that alternating, or humcurrent, passes through this leakage resistance from heater to cathodeand thence to ground. The cathodes of discharge devices I, l and 8 aregrounded except for the impedance of their bias resistors,and thus humvoltage across this small impedance is low enough in value not to causeobjectionable hum. The cathode of amplifier 3, however, is connected toground through its bias resistor. I! and the resistor 5, which latterresistor, since it, in parallel with resistance 21, is the anodecoupling impedance between device 3 and amplifier 8 is so proportionedwith respect to resistance 2I that this, coupling impedance is of valueequal to that of resistance 4. The cathode of this de vice is thuseffectively removed from ground by an impedance equal to. the impedancebetween the anode and ground. A considerable leakage current of the humfrequency may fiow between the cathode 28 of this discharge device andits heater 29 because of the leakage resistance therebetween. Thesecurrents in turn flow around the circuit comprising resistance l1,resistance 5, and the heating conductors 30, thereby setting upundesired hum voltages on resistance 5 which are amplified by amplifier8. Unless an exactly equal voltage of like phase is produced on resistor4 these currents are reproduced in the output circuit and causeobjectionable hum.

It has been found, however, that with the system connected as shown, theparallel resistance of resistances 20 and 23 may be so proportionedrelative to the internal anode impedance of discharge device I and thegain of amplifier 3 that a hum voltage is produced on resistance 4 whichis equal to that produced on resistance 5, where- To accomplish thisresult vice 3 to such an extent that after amplification by amplifier 3it produces the same magnitude of voltage in the anode circuit of thisamplifier. This latter voltage is divided equallybetween resistances 4and 5 and since the portion which appears on resistance 5 opposes thatinitially produced there by the cathode leakage currents the remainingVoltage equals that which appears on resistance 4. The remaining voltageon resistance 5 has the same phase as, and half the mas- 'nitude of, theinitial voltage which appears there and is such that the anode andcathode of this device oscillate in like phase with respect to ground atthe hum frequency.

Since the total output hum voltage of ampli- 15 fier 3 must equal theinitial hum voltage ofresistance 5, the hum voltage supplied betweencathode andgrid of amplifier 3 mustequal this initial hum voltagedivided by-the gain of ampliffier 3. This grid to-cathod'e'voltage isderived 9. from the half initial hum voltage existing across--resistance 5. Thus the parallel resistance of resistors 20 and 23 mustbe so proportioned with respect to the internal impedance of discharge.device I that, of the half'initial hum volta e existing across resistor5, a portion equal to the 7 initial hum voltage divided by tube gain issup plied between grid and cathode of amplifier The resistance ofresistors 20 and 23 "must," therefore, be proportioned in accordancewith the following relation: a

% initial hum voltagex R, 1n1t1al. hum voltage RII+RP R,,- & R 9

Where R2 is the parallel resistance of resistor'sff 20 and 23, Rp is theinternal anode resistance of discharge device I and g is theamplification of amplifier 3. It has been found when so pro-, portionedthe hum voltages produced by leakage currents between the cathode andheaterof discharge device 3 appear with equal intensity and like phaseupon the grids of amplifiers I and 8 a and thus are effectivelyneutralized or balanced out in the transformer 9. This latter conditionrequires that the resistance 20, and 23fbe n'i'adeas small aspracticable and the internalre'sistance of amplifier I. as large aspracticable, these'being' the conditions necessary to the reduction ofdegeneration as previously explained. 'Thus both, of the two resultssecured by my invention, name,- ly the reduction of degeneration and theelimina tion of hum due to these leakage currentsfar'ef effectivelysecured.

While the values of the various resistances. and capacitances involvedmay vary widely, one, set of values which have been. found satisfactoryare as follows Resistance 20-. ohms 180-,000 Resistance 23 do 180,000Resistance 21 do- 47,000 2 Resistance 5 dol 39,000- Resistance 4 do22,000 Resistance 26 megohms -2.2 Condenser 19 microfarad .05 Condenser22 do .1

The discharge device I was-of 638G type and discharge device 3. was of'the 6J5G type,

While I have shown; but a single embodiments of my invention it will ofcourse, bepnderstood1 75,,

2,215,439 thatI do-not wish to be limited thereto since;

different modifications both in the circuit atrangement and in theinstrumentalities employed may be made, and I contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, an electron discharge amplifier having a grid, ananode, and a cathode, a balanced output circuit connected between saidanode and cathode, an unbalanced input circuit connected between saidgrid and the midpoint of said output circuit and a connection from saidcathode to an intermediate point on said input circuit, the impedance ofsaid input circuit between said grid and said intermediate point beingproportioned to reduce degeneration due to voltage supplied from thecathode side of said output circuit to the grid through the impedance ofthe input circuit.

2. In combination, an electron discharge amplifier having a grid, ananode, and a cathode, a balanced output circuit connected between saidanode and cathode, an unbalanced input circuit connected between saidgrid and the midpoint of said output circuit, and a connection from saidcathode to an intermediate point on said input circuit, the impedancebetween said intermediate point and grid being proportioned relative tothe impedance of said input circuit and the gain of said amplifier tominimize the voltage across said output circuit produced by undesiredoscillation of said cathode with respect tosaid midpoint of saidbalanced output circuit.

3. In combination, an electron discharge amplifier having an anode, acathode, and a grid, an unbalanced input circuit, a balanced outputcircuit, said cathode being connected to one side of said input circuitand to the midpoint of said output circuit through an impedance, theopposite sides of said output circuit being connected respectively tosaid cathode and anode, and the opposite side of said input circuitbeing connected to said grid, an impedance connected across said inputcircuit having an intermediate point connected'to said cathode, theimpedance of said input circuit being sufficiently high and theimpedance between said intermediate point and grid suificiently low tominimize the reduction in gain of said amplifier caused by degenerationdue to said first mentioned impedance.

4. In combination, an electron discharge amplifier having an anode, acathode and a grid, an unbalanced input circuit, a balanced outputcircuit, said cathode being connected to one side of said input circuitand to the mid-point of said output circuit through an impedance, theopposite sides of said output circuit being connected respectively tosaid cathode and anode, and the opposite side of said input circuitbeing connected to said grid, energizing means for said cathodeefiective to produce undesired hum voltage across said impedance, animpedance connected across said input circuit having an intermediatepoint connected to said cathode, the impedance between said intermediatepoint and grid being of such value that the voltage produced betweensaid cathode and said grid equals the intensity of said undesired humvoltage divided by the gain of the amplifier.

5. In combination, an electron discharge amplifier having an anode, agrid, a unipotential cathode and ground due to leakage current from saidheater, and means to produce voltage on said anode with respect toground due to said leakage current equal to the effective voltagebetween said cathode and ground.

6. In combination, an electron discharge amplifier having an anode, agrid, a unipotential cathode, and a heater for said cathode, said anodeand cathode being connected to ground through equal impedances, an inputcircuit connected between said grid and ground, a source of alternatingheating current connected to said heater, said heater and cathode beingso arranged that voltage is produced between said cathode and ground dueto leakage current from said heater, and means to supply a sufiicientportion of said voltage to said grid to produce on said anode withrespect to ground a voltage equal to the effective voltage on saidcathode with respect to ground caused by said leakage curtion of thevoltage of said second impedance is supplied between said grid andcathode through said first mentioned discharge device to producedegeneration, and means to increase the impedance of said firstmentioned discharge device to minimize said degeneration.

8. In combination, an electron discharge device having an impedanceconnected between its anode and cathode, an electron discharge amplifierhaving a grid connected to said anode, a cathode connected to anintermediate point on said impedance, and through a second impedance tosaid first mentioned cathode, and an anode connected to said firstmentioned cathode through a third impedance equal to the impedancebetween said cathodes, whereby voltage produced on said first impedanceis amplified by said amplifier and supplied equally to said second andthird irnpedances, and whereby a portion of the voltage on said secondimpedance is supplied between said grid and cathode through said firstmentioned discharge device, alternating current heating means for saidcathode of said amplifier operating to produce hum currents in saidsecond impedance, the proportions of said first impedance on oppositesides of said intermediate point and the impedance of said firstmentioned discharge device being chosen substantially to equalize thehum voltages on said second and third impedances.

9. In combination, an amplifier having an anode, a cathode and a grid,said anode and cathode being connected to ground through equal cathodeand a heater-for said cathodeysaid; anode and cathode being connectedto.ground; through equal. impedances, ,an inputcircuit connected betweensaid grid and ground, a source of alternating heating currentconnectedtosaid heater, said heater and cathode being so a ran at. l a io ed:- be n aid' 1 nating energizing current for said cathode operatingto produce hum currents across said impedance between saidcathode andground, and

means to minirnizedege'neration in said amp lifiei" produced by saidlast mentioned impedance and simultaneously to; equalize voltagesproduced acrosssaid equal impedances due to said energi-zing currents.

CHARLES S. ROOT.

